1. Technical field of the Invention
This invention relates to a battery voltage monitoring device for equalizing cell voltages of a plurality of battery cells connected in series.
2. Related Art
There is conventionally known a voltage equalization device for equalizing cell voltages (or terminal voltages) of a plurality of battery cells (or electricity storage elements) connected in series to form a battery pack. Japanese Patent Application Publication No. 2007-12407 discloses such a voltage equalization device. Specifically, the disclosed voltage equalization device comprises a transformer including: a primary winding, one end of the primary winding being connected to a negative terminal of the battery pack via a switch and the other end of the primary winding being connected to a positive terminal of the battery pack; a plurality of secondary windings one-to-one corresponding to the plurality of battery cells; and an energy storage section (an air gap) formed by utilizing an iron core.
In the above disclosed voltage equalization device, when the switch is turned on, a primary current flows through the primary winding of the transformer and then magnetic energy is stored in the energy storage section. Upon turn-off of the switch, the stored magnetic energy is released from the energy storage section in a form of secondary currents flowing through the respective secondary windings as a function of the cell voltages of the corresponding battery cells, thereby charging the respective battery cells. A larger secondary current flows into a battery cell having a lower cell voltage. That is, the cell voltages of the plurality of battery cells can be equalized among the battery cells sharing the primary winding of the transformer by repeatedly turning on and off the switch.
However, in an electrical vehicle (EV) or the like which has a number of battery cells connected in series to form a battery pack, there exists a disadvantage that an overall circuit size becomes larger with an increasing number of battery cells. To reduce the circuit size, the battery cells connected in series may be grouped into a plurality of blocks to make a voltage across each block lowered. However, a plurality of energy losses or heat conversion losses may occur due to repetitive turning on/off of the switch during a charge transferring process between different blocks, which may lead to significant reduction of charge transfer efficiency. Therefore, when a charge transferring process is performed between mutually distant battery cells with a predetermined number or more of intermediate battery cells therebetween, the process is accompanied by many times of turning-on/off of the switch, which may make the charge transferring process between mutually distant battery cells inefficient.
In the above, the voltage equalization device of transformer type has been explained. A charge-pump-type voltage equalization device and a flyback-type voltage equalization device have a similar disadvantage. In the charge-pump type voltage equalization device, a voltage equalization process is performed through charge transfers between adjacent battery cells within one block or a predetermined number of battery cells and charge transfers between different blocks. However; a charge transfer from one battery cell in one block to another battery cell in another block is accompanied by many times of turning on/off of the switch(es), which leads to reduction of charge transfer efficiency. On the other hand, in the flyback-type voltage equalization device, the voltage equalization is accompanied by sequential charge transfers between adjacent battery cells. Therefore, in the flyback-type voltage equalization device, transferred charge is likely to be eliminated during the sequential charge transfers from one end to the other end of the battery pack or battery block due to the energy losses or heat conversion losses that may occur upon turn on/off of the switch(es).
In consideration of the foregoing, exemplary embodiments of the present invention are directed to providing a battery voltage monitoring device for equalizing cell voltages of a plurality of battery cells connected in series to form a battery pack, which can perform an efficient charge transfer from one battery cell to another battery cell that is distant from the first battery cell while suppressing increase in circuit size of the device.